One means for reducing the overall diameter of a fuel injector comprises using hermetic laser welds instead of O-ring seals at certain joints. This allows certain individual parts which are typically metallic and tubular in shape to be of smaller diameters. The electromagnetic coil assembly that is used for operating the fuel injector must also be made smaller in diameter in order to achieve the desired reduction in overall diameter of the fuel injector. But in order to maintain injector performance, the effectiveness of the coil assembly must not be compromised in the process, meaning for instance that the number of ampere-turns of the coil should not be reduced. Consequently, a reduction in the diameter of the coil assembly might have to be at the expense of an increase in overall length for the coil assembly. Such an increase in length may not necessarily be objectionable, but when accompanied by reduction in the diameter of the coil assembly, it may have a definite influence on other constructional aspects of the fuel injector and/or on the sequence in which various parts are assembled during the injector fabrication process.
The present invention relates to both a novel construction of, and a novel process for fabricating, a solenoid operated fuel injector that enables a smaller overall diameter to be realized through the use of laser welding without sacrificing injector performance. Briefly, the invention comprises providing the through-hole in the non-ferromagnetic bobbin of the electromagnetic coil assembly with respective relatively larger and relatively smaller diameter portions, and also providing the stator that passes through the bobbin through-hole with respective relatively larger and relatively smaller outside diameter portions. In a top-feed fuel injector, the stator that passes through the bobbin through-hole is the fuel injector's ferromagnetic fuel inlet tube, and this fuel injector will be the example used to disclose the invention in the ensuing description.
The relatively larger outside diameter portion of the fuel inlet tube of such a top-feed fuel injector is disposed between the fuel inlet opening at one end of the fuel inlet tube and the relatively smaller outside diameter portion. The relatively larger diameter portion of the bobbin through-hole is at the end of the electromagnetic coil assembly that is toward the fuel inlet opening of the fuel inlet tube. The fuel inlet tube and the electromagnetic coil assembly are assembled by inserting the end of the fuel inlet tube that is opposite the end containing the fuel inlet opening into the relatively larger diameter portion of the bobbin's through-hole and passing the fuel inlet tube through that through-hole until the larger diameter portion of the through-hole comes into press-fit engagement with the larger outside diameter portion of the fuel inlet tube. As the smaller outside diameter portion of the fuel inlet tube was passing through the larger diameter portion of the bobbin's through-hole during initial insertion, it eventually reached the smaller diameter portion of the through-hole. The smaller diameter portion of the through-hole is just slightly larger than the smaller outside diameter portion of the fuel inlet tube so that it acted to coaxially guide continued passage of the fuel inlet tube through the bobbin's through-hole until the aforementioned press-fitting was attained. The fuel inlet tube has a sufficient overall length that a certain amount of the smaller outside diameter portion of the fuel inlet tube protruded beyond the smaller diameter portion of the bobbin's through-hole when the aforementioned press-fitting occurred. The respective transitions between the larger and smaller portions of the bobbin's through-hole and between the larger and smaller outside diameter portions of the fuel inlet tube are in the form of complementary tapered shoulders that are adapted to mutually abut and define the extent to which the bobbin and the fuel inlet tube can be axially press-fitted, and when such abutment occurred, the amount by which the smaller outside diameter portion of the fuel inlet tube protruded from the smaller diameter portion of the bobbin was established. This amount is chosen to be sufficient to provide for a short neck of a non-ferromagnetic metal shell to be telescoped over a neck at the protruding end of the fuel inlet tube and joined thereto in a hermetically sealed manner, preferably by laser welding, so that the outside of the neck of the non-ferromagnetic shell was flush with the outside of the smaller diameter portion of the fuel inlet tube. The aforementioned press-fit of the bobbin on the fuel inlet tube assured that the electromagnetic coil assembly would be held clear of the weld zone during welding of the non-ferromagnetic shell to the ferromagnetic fuel inlet tube. After welding, the electromagnetic coil assembly was displaced axially relative to the fuel inlet tube to break the press-fit of the bobbin from the fuel inlet tube and bring the smaller diameter portion of the bobbin's through-hole into covering relation to the laser weld, and the electromagnetic coil assembly was axially located in a desired final position by its abutment with a shoulder of the non-ferromagnetic shell that extends radially outward from the non-ferromagnetic shell's telescopic engagement with the fuel inlet tube.
This novel construction intentionally precludes the possibility of assembling the fuel inlet tube and the smaller diameter coil assembly by first inserting the inlet end of the fuel inlet tube into the smaller diameter portion of the bobbin's through-hole, but by precluding this possibility, it provides a novel process for fabrication of a smaller diameter fuel injector that is embodied in the scope of the present invention. Since the presence of the shoulder of the non-ferromagnetic shell is necessary in this particular fuel injector, such a smaller diameter electromagnetic coil assembly could not be used if it were necessary for the non-ferromagnetic shell to be joined to the ferromagnetic fuel inlet tube before the coil assembly had been placed onto the fuel inlet tube since the diameter of the bobbin's through-hole would have to be large enough to fit over the shoulder of the non-ferromagnetic shell, and as a consequence, the overall diameter of the coil assembly would have to be made larger too. The particular sequence of steps described herein therefore constitutes an inventive process aspect for fabricating a fuel injector.
Various features, advantages and the inventive aspects will be seen in the ensuing description and claims which are accompanied by drawings that disclose a presently preferred exemplary embodiment of the invention according to the best mode contemplated at the present time for carrying out the invention.